Speak with him to learn only one NPC can accompany you on this journey, so pick who you think will help you best, then step through the portal. Proceed through the tunnel, avoiding the traps, and you will find Sai and a Doppelganger. The Doppelganger can be tough so make sure to stun and block and power attacks, or else you may go down in one blow. Once it is killed, Sai will get pulled away from you, forcing you to continue forward.

Fight through the skeletons to reach the next area, where you will find another Doppelganger protected by a shield. Go to the four crystals, kill the Daedra guarding them, then deactivate the crystals. Once down, engage the Doppelganger. She is fairly weak at first, but once she shifts form, she will employ a powerful AoE attack. Make sure to stun it quickly, or to keep your distance from her when she uses it. The orbs that float towards her also need to be killed as they heal her, however if you destroy them, they heal you instead. Once dead, Sai will get pulled away again so you must push forward.

Once you reach the last room, take down the Varen Doppelgangers and the final boss will appear. She is surprisingly easy compared to the previous fights, so dispatch her quickly then free Sai. Now return to Tamriel and speak with the group to finish the quest.